So, my first board game design project is a Co-operative, Worker Placement, Fighting, Political game with the intent of having up to 4 players and I wonder if I should take that into account right from the start or not. Is there a suggested number of players when you start designing a game in order to make your life a bit easier when you try to deal with math and balancing issues? Will it be easier in general if you start with a solo variant at any case? Is this reasonable enough when you know right from the start that you want to make a co-op worker placement?

Solo variants often need to be a little different in order to function properly (use of Automa cards, etc), so personally I wouldn't start there unless I was designing a primarily solo game.

I tend to have 2-4 in mind as I start, but then I've not got very far with any of my designs! Most of mine tend to be Euro-style games with limited or indirect interaction; yours sounds a little more confrontational or directly interactive, so hard to say.

But then it's Co-op, so again, hard to say! Probably for a Co-op I'd start with the 4, then work out how 3 and 2 players will play it to give the same experience?

The wider the range of number of players that can play and have a good experience with your game the better.

A turn based game usually gets too long (too much downtime) beyond 4 or 5.

A game with hidden information between players is hard to design for 1.

There are not many fast games for groups of more than 5.

Auction, negotiation, traitor and political games usually need at least 3, perhaps 5 or more to really shine.

So your game mechanics will dictate how many players your game can accommodate. But pick a number and design mostly for that number. The bulk of your time should be spent perfecting the game for that number. But keep in mind other numbers also and how you will adjust the game for them. In general design for the high number of players since that is where the game will take the longest (and thus you will be encouraged to shorten the play time).

Co-op games without hidden information, usually scale well. They are usually easy to play solo.

For a co-op one question is how different are the roles of the different starting players. If 'not different at all, just think of them as red shirt, vs yellow shirt' then you have a different problem than a game that has Variable Player Powers. If you have Variable Player Powers, for the game to work with 2, you will need to see that the game is winnable with each possible starting pair combination, but it may be perfectly fine if some combinations are a lot harder than others. Indeed this may be where the replayability comes in. But clearly mark this in your rulebook or risk the wrath of those who think that 'being unbalanced' is a design flaw, rather than, in some cases, especially with co-ops, a feature.

Solo gamers do not demand that the game is solvable with each starting character, alone, though they may be very happy if your game provides this. 'It's a co-op so play running however many characters gives you the most fun' is the more general rule.